Chords and Scales
Unlike the major scale, the harmonic minor has no exact equivalent in the older system of church modes. Both the Aeolian and Dorian modes contain the characteristic group of four notes that begin the minor scale (labelled y). If tonal music was logical rather than practical, the Dorian mode, with its division of the octave into two identical groups, would be the obvious minor equivalent to the major scale.
The minor scale is a good example of how tonal music creates tonality rather than the other way round. What makes it confusing from a theoretical point of view is that it changes according to context. Theorists in the past have perhaps worried too much about trying to present the tonal system as logical; the minor scale is not a product of theoretical logic but of practical usage.
The Aeolian mode is sometimes called the 'natural minor', but this is not in fact the group of notes that most minor key music is based on. Chords in minor key pieces tend to draw their notes from the harmonic minor scale in the example below. However, composers tend to avoid writing melodic lines that contain the augmented second of the harmonic minor. The second example below shows how it is often altered to produce a smoother flow. The modifications are different for ascending and descending melodies.
Although these modifications make some melodic sense, you have to wonder why the harmonic minor scale contains an augmented second in the first place ...
To understand the reason why the seventh is sharpened, you have to look beyond a theoretical discussion of scales and triads to the sorts of chord progressions on which the rest of this reference guide concentrates. In particular you have to understand a little about the history of cadences.
The examples below show two types of cadence based on the natural (Aeolian) and harmonic minor scales, with the seventh degree highlighted in red in each case. Those marked a) are typical of vocal music before 1600, while those marked b) can be found in a wide range of music since. Listen carefully to the difference between two versions. The Aeolian may sound a little strange because cadences without the seventh scale degree sharpened are so unusual.
||Cadences in which the upper voice finished with the ascent of a tone (like the Aeolian version) were increasingly considered less final than those in which the interval in the upper voice was a rising semitone. The Ionian mode (the predecessor of the major scale) naturally finished in this way but it became more and more common for the seventh to be sharpened in modes where this was not the case.
||As the common practice major-minor system became more established in the seventeenth and eighteenth centuries, the preference for rising semitones at cadences persisted. As a result, chord V in perfect cadences is invariably a major triad so that the seventh degree rises by semitone to the tonic.
Because the sharpened seventh degree of the scale is said to 'lead' more finally to the tonic, it is often called the leading note. As explained elsewhere on TonalityGUIDE, the leading note is very important for establishing the sense of 'being in a key' and therefore also for modulations.
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© Copyright Thomas Pankhurst